How do I calculate a miter angle?

Question

See the image below:
When I lay this out in Fusion, I can get the angles but I can't figure out how to calculate it mathematically.
The goal is to have the angled stretcher fully contact the upper 45"-long support and the floor(even though there will be excess to remove.)
I'm looking for a formula for calculating the stretcher angle (35.8 in this case) and the length that the 2x4 would have to be.
I know how to calculate all of the angles of a right triangle, but no combination or the dimensions of the project give the correct angle.

user138719 · Answer

Here's a solution I wrote over on the engineering site where this problem is cross-posted.
For anyone who may want to actually use this formula without evaluating the trig, here is a worksheet where you can enter the width of board you want to use, together with the length and height of the rectangle you need to span, and it finds a cut angle and the length of board you'll need.

How do I calculate a miter angle?

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